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We have a Cpanel server with lots of accounts on it. Most of the users have wordpress accounts. Last week one WordPress site was showing a HTTP 500 error and the site was offline. It only shows an empty page/500 Error page. No other users have any issue on the same server.
We did restore the cpanel account backup - and the page was up again.
The next day the site was just showing HTTP 500 Error again.
We did then restore the cpanel backup and the site was online again. We did change the cpanel and FTP password. We did also change the wordpress admin passwod we got from the customer.
The next day the site had the HTTP 500 error again.
We restored the backup, change wordpress admin password. We did then update wordpress and all plugins/themes for the customer. (some was around one year old) We did not give the customer any of the new passwords.
The site then worked fine Wednesday, Thursday, Friday but on Saturday the site got the 500 Error message again.
On Monday we restored the backup, now also disabling their guestbook - since we could see in the site logs that the last log entry before the 500 Error message was an guestbook entry.
Tuesday the site was offline again. We restore the backup, and one more time I disabled the guestbook, and also disabled commenting in Wordpress, and also deleted around 19.000 SPAM comments.
The site worked fine from Tuesday to yesterday. But today the site has the 500 HTTP error again.
We have also let the webdesigner of the site look at all source code on Wordpress and all plugins and they says that they do not see any issues with the source code.
Why do this happen, not every day, but several times per week? Why do a account restore in cpanel fix the issue, without doing anything on the server? Is it some kind of hacking of the WordPress installation? Is it a server issue, if yes, why is only one of around 200 sites affected?